Proteins

> nitrogen Nitrogen is found in proteins but not in carbohydrates and lipids. Proteins, carbohydrates, and lipids all contain the elements carbon, hydrogen, and oxygen. Proteins are made of amino acids, one component of which is a nitrogen-containing amine group.

> deoxyribonucleic acid (DNA) Protein synthesis is directed by a compound in the cell nucleus called DNA. DNA stores genetic information as genes, segments of DNA that direct the production of proteins. Although DNA provides the code for proteins, it doesn't actually take part in the building. Therefore, other important molecules are needed. Messenger RNA transcribes the code from the DNA and brings it out of the nucleus and into the cell cytoplasm, to the ribosome. There, the code is translated into the "language" of amino acids. Specifically, transfer RNA brings specific amino acids to the ribosome for protein assembly.

> Black beans Black beans eaten with brown rice will ensure all of the body's amino acid needs are met. When incomplete proteins such as brown rice are coupled with modest amounts of animal proteins or soy, or are combined with other plant proteins that are rich in the incomplete protein's limiting amino acids—such as beans, the incomplete protein becomes a complete protein. Broccoli, onions, and orange slices do not provide the amino acids limited in brown rice and other grains.

> there is a severe deficiency of protein in the diet but an adequate intake of energy Kwashiorkor develops when there is a severe deficiency of protein in the diet but an adequate intake of energy. With protein deficiency, fluid accumulates in the spaces surrounding the cells, causing edema (swelling) in the legs, feet, and abdomen. Muscle proteins are broken down to generate the amino acids needed to synthesize critical functional proteins. Animal protein in the diet is not necessary, because people can meet their protein needs from a combination of plant proteins, such as soy and other legumes, grains, and vegetables, as part of a well-balanced diet. Marasmus is a condition in which there is a severe deficiency of energy intake, including protein intake.

> buffers Proteins act as buffers and help to maintain proper pH in blood by picking up or donating hydrogen ions. Typically, blood has a pH of 7.4. Even slight changes in this pH can be dangerous and even life-threatening. When blood pH is too basic, side chains on amino acids donate hydrogen ions to the blood. If the blood becomes too acidic, amino-acid side chains can grab onto excess hydrogen and reestablish acid-base balance. Enzymes are substances that act as catalysts and speed up chemical reactions without being used up in the process. Hormones are chemical messengers that elicit a specific response in the body. Catalysts are substances that help to speed up chemical reactions in the body.

> transporting nutrients through the bloodstream or across cell membranes Proteins play many important roles in the body, including transporting nutrients through the bloodstream or across cell membranes. Adequate amounts of protein are needed to transport certain nutrients, such as triglycerides, through the bloodstream and to shuttle certain vitamins and minerals, as well as oxygen and waste products, into and out of cells through cell membranes. Calcium and other minerals—not proteins—mineralize bones. The mineral iron binds to and transports oxygen within red blood cells.

> amine group The part of the protein molecule that contains nitrogen is called the amine group. In addition to the amine group (NH2), each amino acid that makes up a protein has an acid group (COOH), a side chain, a hydrogen atom, and a central carbon. All amino acids contain the same parts, and the side chain is what makes each amino acid different.

> 20 percent An 8-ounce glass of low-fat chocolate milk that contains 160 kcal and 8 grams of protein has 20 percent of its kcal from protein. Recall that protein provides 4 kcal per gram. A food or beverage with 8 grams of protein contains 32 kcal from protein (8 grams of protein x 4 kcal/gram = 32 kcal). The protein percentage of total kcal is thus 20 percent (32 kcal from protein ÷ 160 total kcal = 0.20, or 20 percent).

> refrigerating milk Refrigerating milk does not alter the bonds between the amino acid side chains in the milk proteins, and so it does not denature the proteins. Heating a protein (such as when grilling a chicken breast or frying an egg) and adding acid to it (such as marinating a steak in acidic red wine) will denature proteins by altering the bonds.

> small intestine After a meal, the majority of protein digestion and absorption occurs in the small intestine. Chemical digestion of protein begins after chewed food enters the stomach. Hydrochloric acid breaks down the bonds of protein strands and activates pepsin, a digestive enzyme, which breaks the protein into shorter polypeptide strands. In the small intestine, proteases further break down protein strands into tripeptides and dipeptides, as well as some amino acids. The protein remnants are then absorbed into the cells of the small intestine lining, where the remaining tripeptides and dipeptides are broken down into single amino acids that are then released into the bloodstream. In the mouth, mechanical digestion of protein begins with chewing, tearing, and mixing of food with salivary juices to form a bolus, or soft mass of chewed food. The pancreas produces enzymes, including proteases, that are released into the small intestine via the pancreatic duct.

> 10 to 35 percent The AMDR for protein is 10 to 35 percent of one's daily energy intake. Currently, adults in the United States consume about 15 percent of their daily energy from protein, which falls within this range. Carbohydrates should comprise 45 to 65 percent of an individual's daily kcal intake, and fat should comprise 20 to 35 percent. Consuming these nutrient types in these ranges will ensure that energy and nutrient needs are met and will reduce the risk of developing chronic disease.

> denaturation The process that destroys the shape and activity of proteins when heat, acid, salts, or pressure is applied is known as denaturation. This process disrupts the bonds between the amino acid side chains, causing the protein to uncoil. A hard-boiled egg, whipped cream, and curdled milk are all examples of denaturation. Condensation is a chemical reaction by which smaller molecules join to form larger compounds and water is released as a byproduct. Hydrogenation is the process in which hydrogen is added to an unsaturated fatty acid to make it more saturated and solid at room temperature. Deamination is the process by which an amine group is stripped from an amino acid.

> meat A complete protein is a protein that provides all the essential amino acids that a body needs, along with some nonessential amino acids. Complete proteins are found abundantly in animal sources, such as meat, fish, and poultry. Soy protein is also a complete protein. While there is some protein in vegetables and in whole grains, this protein is considered incomplete because it is low or deficient in at least one of the essential amino acids. Fats do not contain protein.

> 9 Of the 20 unique amino acids found in protein, 9 are considered essential to consume in the diet, because the human body cannot make them. The remaining 11 amino acids are just as important to human health, but they are called nonessential amino acids because they can be synthesized, or created, in the body. It is not essential to consume them in the diet.

> 56 grams Charlie should consume 56 grams of protein daily. The current RDA for protein is based on age and weight. Nonathletic adults age 19 and older should consume 0.8 gram (g) of protein for each kilogram (kg) of body weight. Since Charlie weighs 70 kg, he should consume 70 kg × 0.8 g, or 56 grams of protein a day.

> Transfer RNA collects amino acids from the cytoplasm and brings them to ribosomes for assembly into proteins. During protein synthesis, transfer RNA collects amino acids from the cytoplasm and brings them to ribosomes for assembly into proteins. Only the first step of protein synthesis—transcription of the DNA to messenger RNA—occurs in the cell nucleus. Messenger RNA then leaves the nucleus, and carries its genetic message to the ribosomes within the cytoplasm. There, the information is translated into a sequence of amino acids, which transfer RNA collects from the cytoplasm and brings to the ribosomes.

> All options are correct. All options are correct. After the digestive process is complete, absorbed amino acids are degraded, with the nitrogen converted to ammonia and then to urea and excreted in urine. The remnants are then used for energy, or are stored as fat. When the diet is too low in carbohydrates, amino acids are used to make glucose to supply energy to the brain. Surplus amino acids from excess dietary protein are stored predominantly as fat. A limited supply of all amino acids remains in amino acid pools in the blood and inside the cells. Because the body can't make the essential amino acids, the pools need to be constantly restocked. Amino acids are continually being removed from the amino acid pools to create proteins on demand. Many of the absorbed amino acids are used to build and maintain body tissues and functional chemicals such as enzymes and neurotransmitters.

> positive nitrogen balance A body that retains more nitrogen than it excretes is in positive nitrogen balance. Rapidly growing babies, children, and teenagers are all in positive nitrogen balance because their bodies use nitrogen to build new tissues that aid growth, build muscles, and expand the supply of red blood cells. Pregnant women are also in positive nitrogen balance. Nitrogen balance is the state in which an individual is consuming the same amount of nitrogen (from protein) in the diet as he or she is excreting in the urine. Healthy, nonpregnant adults are typically in nitrogen balance, or equilibrium. Negative nitrogen balance occurs when the body excretes more nitrogen than is consumed. This usually occurs due to a physical impairment such as a serious illness or injury, malnutrition, or other trauma. As a result, the body cannot synthesize protein as quickly as it's broken down.

> chicken breast A chicken breast is a source of complete protein. A protein that provides all nine of the essential amino acids, along with some of the 11 nonessential amino acids, is considered a complete protein. Protein from animal sources, such as meat, fish, and poultry, is typically complete protein. Soy, a plant protein, is also a complete protein. A protein that is low or deficient in one or more of the essential amino acids is considered an incomplete protein. Protein from plant foods (such as vegetables, grains, and beans) tends to be incomplete; however, you can obtain a complete protein by combining complementary foods, such as rice and beans.

> All options are correct. All options are correct. Proteins are involved in most of the body's functions and life processes and are necessary for survival. A major function of protein in the human body is to maintain fluid balance. Proteins play a major role in ensuring that body fluids are evenly dispersed in the blood and inside and outside cells. Proteins are the body's building materials and are needed for the ongoing growth and maintenance of new tissue. Proteins are also needed for the synthesis of protein and amino acid hormones that direct specific activities. For example, the hormones insulin and glucagon regulate blood glucose level.

> pork Pork has the greatest digestibility of these proteins. Protein quality is determined by its digestibility, or the body's ability to digest it, and by its amino acid composition. Generally, animal protein is about 99% digestible, while plant protein (including peanuts, beans, and spinach) is only 87% digestible. Some plant protein is protected in the plant's cell walls, which cannot be broken down by the human body. Therefore, some protein is unavailable for use and is excreted from the body. Cooking plant protein can help to break down the cell walls and make plant protein more digestible.

> vitamins D and B12, calcium, and iron Unless carefully planned, vegan diets may lack vitamins D and B12, calcium, and iron. These vitamins and minerals, as well as protein, zinc, vitamin A, and omega-3 fatty acids, are abundant in animal-based foods, which a vegan diet does not include. Animal-based foods are the only naturally occurring food source of B12, so it is extremely important that vegans consume fortified foods or a supplement to meet their daily needs. A well-planned vegan diet that includes a range of fruits, vegetables, legumes, grains, and vegetable oils provides ample amounts of vitamins C, E, and K, thiamin, and folate, as well as sodium, potassium, and magnesium. Although vitamin A is found only in animal-based foods, vegans can get adequate amounts by consuming beta-carotene, a plant pigment that is converted to vitamin A in the body.

> antibodies A healthy immune system depends on an adequate supply of antibodies. Antibodies are proteins made by the body to bind to and neutralize foreign invaders, such as harmful bacteria, fungi, and viruses, as part of its immune response. Enzymes are substances that act as catalysts and speed up chemical reactions. Buffers are substances that help maintain the proper pH in a solution by attracting or donating hydrogen ions. Transport proteins are proteins that carry lipids (fat and cholesterol), oxygen, waste products, and vitamins through the blood to various organs and tissues or that serve as channels to allow substances to pass through cell membranes.

> peptide bonds Proteins consist of a chain of some combination of amino acids linked by peptide bonds. In peptide bonds, the amine group of one amino acid is bonded to the acid group of another. Each protein contains a chain of at least 50 amino acids, though proteins typically contain between 100 and 10,000 amino acids. Double carbon bonds are found between carbon atoms, for example in unsaturated fatty acid chains. Phosphate bonds link phosphate groups, for example in ATP. Denaturation is a process in which proteins lose their shape, for example when exposed to high heat or acids.

> soy milk Soy milk is a source of complete protein. Although plant protein is typically not complete protein, soy is an exception. Soy foods, such as soy milk, provide all the essential amino acids a body needs, along with some nonessential amino acids, and thus are a source of complete protein. Protein from animal sources, such as meat, fish, and poultry, is typically complete protein, whereas protein from plant foods tends to be incomplete. Kidney beans, peanut butter, and pasta are missing adequate amounts of some essential amino acids.

> All options are correct. All options are correct. Stir-fried tofu and vegetables over brown rice; a cheese and broccoli omelet; and baked ziti with ricotta cheese, spinach, and tomato sauce are all acceptable main dishes for a lacto-ovo-vegetarian. Lacto-ovo-vegetarians eat a predominantly plant-based diet that also includes dairy foods and eggs. They avoid meat, poultry, and fish.